Diamond is potentially a very useful material for numerous scientific and engineering applications because it possesses high hardness, has the highest thermal conductivity of any known material, has excellent optical transparency throughout the ultraviolet to infrared range, is chemically inert, has high stiffness and superior acoustical properties, and is biocompatable. Moreover, diamond can be an excellent n- or p-type semiconductor if doped with appropriate impurities.
Because of the unique characteristics of diamond, there are many possible applications of diamond thin films. Some of these are illustrated in the following Table 1:
TABLE 1 ______________________________________ Applications of diamond thin films Characteristic Applications ______________________________________ High thermal conductivity Heat sinks for semiconducting devices, heat dissipating printed circuit boards, and minimum laser damage High hole mobility, dopability n- and p-semiconducting materials, temperature and radiation-resistant power semiconducting devices, transistors in motors and jet engines, Schottky diodes Optical transparency Coatings for lenses and glasses, infrared windows, high density compact disks, X ray windows and lithography masks Wear resistance, hardness, Abrasives, drilling tools, low friction bearings for machines and motors, cutting tools, dies and molds, scratch proof coatings on watches, glasses, ceramics and jewelry Chemical inertness Chemical reactors, inert coatings on nuclear reactor walls, and corrosion-resistant watch cases High sound propagation velocity Loudspeaker diaphragms, tweeters, and midrangers Biocompatability Surface improvement of joints, teeth, implants, prosthetic materials, and biosensors ______________________________________
The most useful form of diamond for the electronics industry would be a film of single-crystal diamond on a nondiamond substrate. Since diamond is both a good thermal conductor and a good semiconductor, diamond microchips could be used successfully in high-speed devices
At the present time, diamonds are synthetically produced for most industrial purposes by using one of two procedures. One method is to apply high temperatures and high pressures to carbon containing compounds. However, this method is not suitable for coating many of the substrate materials for which the full benefits of the unique properties of diamond can be best utilized. The second method is to deposit diamond thin films from the gas phase at low pressures using thermal, electric discharge and catalytic methods. Diamond thin films created in this manner on a substrate material use a variety of chemical vapor deposition (CVD) procedures that include the use of plasma, microwave, hot filament, ion beam, and electron beam as energy sources. Typically, a mixture of 0.5 to 2% methane and the balance hydrogen gas is used as the carbon source. The mechanism of diamond deposition occurs because the hydrogen molecules in the gas decompose into atomic hydrogen which subsequently becomes deposited on the substrate as a monolayer. The more reactive forms of carbon such as graphite combine with the atomic hydrogen and are removed, thereby allowing diamond to condense on the substrate surface.
Although the deposition of diamond films has been proven using the above methods, there are many disadvantages of the prior art procedures that prevent diamond films from being successfully and extensively utilized for commercial applications. Some of these disadvantages are:
1. The deposition rates are generally low. A typical value is 1 micron per hour, although The Naval Research Laboratory claims to have formed "good quality, clear crystals" at 50 microns per hour while the Tokyo Institute of Technology claims rates as high as 930 microns per hour.
2. The deposition area covered on the substrate is small. Current technology is limited to 4" diameter optical surfaces.
3. The substrate temperatures must be in the range from 800.degree. C. to around 1000.degree. C. These high temperatures restrict diamond from being deposited on low melting point substrates such as polymers.
4. The process of diamond growth is not yet fully understood.
5. Hydrogen impurity will be present if the substrate temperature is held below 400.degree. C. when using the ion beam technique. The properties of films containing hydrogen do not approach the desirable properties of diamond films.
6. Adhesion of the film to the substrate is insufficient because of the thermal expansion mismatching of diamond with most of the substrates.
7. A smooth surface finish is hard to achieve especially for optical applications.
Because of the forgoing deficiencies in the prior art methods, new methods for producing diamond thin films have been explored in as effort to overcome at least some of the limitations of the existing methods and to improve the quality of diamond films. One such known method is laser-induced chemical vapor deposition (LCVD). In this method, a laser serves as an energy source to decompose the gases and raise the surface temperature of the substrate for deposition. LCVD offers several unique advantages over the other prior art methods:
1. Laser is a clean source of energy which leads to less contamination for diamond growth. PA0 2. A relatively high deposition rate is achieved. PA0 3. A low substrate temperature can be used. PA0 4. The area of deposition can be controlled. PA0 5. Better surface integrity results. PA0 6. Fine microstructures are produced. PA0 7. The method has the capability for using different precursors. PA0 8. Little thermal damage occurs to the substrate.
Lasers deposit thin films by pyrolysis and photolysis mechanisms of breaking the gaseous molecules. In laser pyrolysis, the substrate is heated by a directed laser beam up to the desired temperature by controlling the power and irradiation time of the beam, while the chemical gases decompose by the collisional excitation with the hot surface. The laser-driven reactions are different from those initiated by other CVD sources for the same heat input because of the higher temperatures obtainable in the smaller reaction volume defined by the directed laser beam. In the laser photolysis process, the photons break the chemical bonds of the gaseous molecules which allows the products to get deposited on the substrate. An important requirement in photolysis is the need to match the wave length of the laser beam with the bandgap of the reactants. The bond breaking may be caused by either single or multiphoton dissociation. In general, pyrolysis is more efficient than photolysis.
Because of these capabilities and characteristics of lasers, some prior art researchers have attempted laser-induced CVD for diamond deposition, and the results initially indicated that diamond films were obtained using an ArF-excimer laser beam in a gas mixture of C.sub.2 H.sub.2 and H.sub.2 at a substrate temperature in the range 40.degree. to 800.degree. C. This seemed to be an important improvement in diamond technology because the deposition of the diamond film was claimed to occur at a temperature as low as 40.degree. C. However, the same researchers later indicated that their earlier findings were not correct and that the film observed was a "heat treated carbon black" instead of diamond.
Other prior art researchers have experimented with a system using a mixture of CCl.sub.4 and H.sub.2 gases that are passed over the substrate and irradiated with a 193 nm ArF laser beam. This gas mixture was apparently chosen instead of CH.sub.4 because it absorbs the 193 nm wavelength more efficiently, the binding energy is more appropriate, and it forms reactive radicals. Using a laser power of 8 watts, it was determined that the maximum growth rate of 6 microns per hour occurred at a substrate temperature of 800.degree. C. It is believed that the surface of the film is photoreactive which activated the hydrogen which in turn increased the amount of carbon available for the diamond film.
Amorphous carbon films have been deposited by excimer laser photodissociation of C.sub.2 H.sub.3 Cl and CCl.sub.4 gases at ambient temperature, and recently, researchers have claimed the deposition of carbon films containing a large fraction of diamond by using an excimer laser and carboxylic acids.
Other than the foregoing examples, there are no known prior art teachings of the successful use of lasers to produce diamond thin films on a substrate, but in light of these prior art teachings, it is evident that laser CVD is a technology that has potential for diamond deposition. However, the prior art LCVD methods all require relatively long processing times and supplementary heating of the substrate in order to attain the substrate temperature necessary to produce diamond deposition. Moreover, the prior art processes must take place in an enclosure to confine the gases from which the diamond film is formed. These are obvious limitations on the use of the LCVD methods. In addition, the prior art teachings have not established optimum laser parameters and other process variables for growing diamond thin films using CH.sub.4 /H.sub.2 precursors, nor does the prior art contain any teaching on the use of lasers other than excimer lasers. There is therefore a definite need for improved methods of utilizing the obvious advantages of laser technology for diamond deposition.